MRI systems are well known in the art and widely used in medicine and typically comprise a magnet system for generating a steady magnetic field, a magnet system for generating gradient fields, and an RF transmitter coil for generating an RF field which excites nuclei in a patient for magnetic resonance. The magnetic resonance signal is detected by the RF transmitter coil or by a special receive-only RF coil or coils. See, for example, U.S. Pat. Nos. 4,859,947; 4,920,318; 5,030,915; and 5,198,768, whose contents are incorporated herein by reference. The present invention is directed to the RF coil or coils for detecting the MRI signals.
Two important factors to evaluate an RF coil are signal-to-noise ratio (SNR) and homogeniety. The SNR is the ratio between the signal strength on the image and the background noise. An RF coil typically achieves a higher SNR when it is closer to the part of the patient being imaged. The homogeniety measures the geometric distortion generated by the RF coil. The receiving sensitivity generally decreases with increasing distance from the coil wires.
In magnetic resonance angiography (MRA) of the lower extremities of a patient, an RF coil is required to cover the blood vessels from pelvis to toe. The region required to be covered by the RF coil is referred to as the field of view (FOV). The commercially available MRI apparatus typically provides an RF coil (sometimes referred to as the body coil) that is large enough to cover the FOV of the lower extremities, but the MRI signals detected with such a coil have too low a SNR.
Another commercially available system uses either a torso flexible phased array coil or a knee coil but their FOVs are too small for the MRI study of the lower extremities. As a result, the MRI technologist has to move the coil and reposition the patient three times to cover the full lower extremities. This costs additional valuable MRI examination time, and is inconvenient for both the patient and the technologist.
A paper in the names of Roemer et al., published in "Magnetic Resonance In Medicine" vol. 16 pages 192-225 (1990), entitled "The NMR Phased Array", describes an array made up of a multitude of overlapping and closely positioned RF coils which can cover a wide region of interest. The example given is of four square overlapped fixed coils for imaging the spine. This system suffers from the drawback that it does not work well for structures deep in the body but is best for near-surface MRI as is needed for spine imaging.
For completeness' sake, a paper in the names of Kojima et al., published in Radiology 1995, 196:871-875, describes an RF phased array coil for imaging the lower extremities. This coil system comprises 6 butterfly type coils, of which each coil comprises a part under the patient and a part over the patient. But this system does not offer the advantage of quadrature operation with its higher SNR.